Post-Tension Concrete

Description

Post-tensioning applications could be applied to buildings designed for different usage such as residential , commercial and office buildings, parking, structures, slabs-on -ground, bridges, sports stadia, rock and soil anchors and watertanks. In many cases, post -tensioning allows construction that otherwise would be impossible in a traditional manner due to either site constraints or architectural requirements.

Post-tensioned concrete slabs have become a major factor on the construction of floor systems for commercial and residential buildings of all types. In their two most popular forms (one way slabs, two way flat plates) they have been found to be economical for structural applications in parking structures, apartment buildings, office buildings, hospitals and industrial buildings of both the high rise and low rise type.

One of the main reasons that led to the development of this technology is that post-tensioning can solve simultaneously weight, deflection and cracking problems, which arose with conventionally reinforced slabs.

Other major factor which contributed to the growth of post-tensioned slab construction included:

Improvements and simplifications in post-tensioning hardware and field methods, making post-tensioned slab construction as easy for the contractor as conventionally reinforced slabs. Improvements in forming systems, which enhanced the overall economics of cast-in place slab construction

Testing programs on post-tensioned slabs, which greatly expanded the understanding of their behavior, and led to improved code criteria and more economical, safe designs While more conventional reinforced is used, both for minimum crack distribution steel and for strength requirements, the combined use of per-stressed and non-per-stressed reinforcement results in considerable savings compared to the previous highly post-tensioned slabs.

The primary reason for the initial economy of post-tensioned industrial floors is the reduced thickness of the concrete slab, permitted because of the compressive stress induced in the concrete by the post-tensioning tendons. Additional reductions in costs are provided by elimination of most slab joints, reduced construction time and, in some cases, the elimination of pile supports or drilled shafts. For floors of prefabricated metal buildings, the post-tensioning also serves as a horizontal tie for the horizontal reaction from the building columns. This eliminates the need for conventional reinforcing bars in the slab to dissipate the column reaction into the slab, and at the same time eliminates the possibility of slab cracking associated with such details.

In addition to economic advantages, present day design criteria for post-tensioned slabs result in improvements in overall behavior (lower compression stresses and more bonded reinforcement reduce and distribute cracking caused by axial shortening restraints.

Such slabs generally have much less cracking than conventionally reinforced slabs. Design criteria and technology for post-tensioned slabs have been developed to a level comparable to that available for conventionally reinforced slabs. In detail, the advantages of the use of post-tensioned slab can be summarized as follows:

Control of Cracks - Post-tensioned slabs control cracks much better than other reinforced slabs, due to the squeezing pressure (tension) of the cables compressing the cracks.

Time Savings - Post -tensioned can be installed much faster that other reinforced foundation systems. A typical residential foundation takes 3-4 hours to install. Time is also saved by, using fewer joints, narrower footings, less digging, and easier to clean up in the event of inclement weather. Slab post-tensioning systems

Reinforcing Savings - As steel and concrete increase in pricing, the savings with a post- tensioned slab will increase, because the quantity of steel and concrete required for a post-tensioned slab is less than for a conventionally reinforced foundation. Clean out of the footings after a rain is much faster and neater than rebar type foundations.

Fewer Joints - Large concrete areas, such as tennis courts, parking lots, warehouses, metal buildings can have joint spacing increased to minimize cost of joints and long- term maintenance of the joints.

Deflection Control - Post-tensioning increases the slab stiffness and by its increased flexural and tensile capacities. For example in presence of expansive soils, it’s obviously that post-tensioned slab are more resistant. Infact, movement of soils can sometimes be significant enough to move the structure (slab on ground support foundation systems are still interactive with the soil).

Improved Modulus of Rupture - It is a fact that concrete shrinks when it dries and cracks. By utilizing a two part tensioning process, shrinkage cracking can be reduced.

Reduction in slab thickness - Minimum floor thickness maximises the ceiling zone available for horizontal services, minimises the self weight and foundation loads, and keeps down the overall height of the building.